This invention relates to a carbon-containing refractory which can be used for nozzles, plates for sliding nozzles, and other items employed in the continuous casting of molten metals. This invention also relates to a manufacturing method for such a carbon-containing refractory.
Presently, refractories for use in continuous casting are generally carbon-containing refractories which employ oxides such as alumina, silica, or zirconia as a main raw material.
A sliding nozzle is used to control the flow of molten metal during continuous casting, and therefore a refractory which is used for a sliding nozzle must have thermal shock resistance, wear resistance, and high mechanical strength.
At present, the carbon-containing refractories which are primarily used for this purpose are made from a combination of oxide-type raw materials, which have excellent corrosion resistance and wear resistance, and carbon materials, which have excellent thermal shock resistance. As a result, they exhibit excellent properties which were not obtainable with conventional oxide-type burned bricks. However, the sintering of the oxide aggregate in carbon-containing refractories inhibits the formation of ceramic bonds, so the mechanical strength of carbon-containing refractories is lower than that of oxide ceramic bonded refractories.
In order to increase the mechanical strength of carbon-containing refractories, it is common to add metallic Al, metallic Si, or the like and perform burning to form carbide bonds or nitride bonds within the matrix (see Japanese Patent Application Laid Open No. 57-27971). Another method which is employed to increase the mechanical strength is to add to a carbon-containing refractory an oxide having a high vapor pressure at high temperatures, such as MgO or SiO.sub.2, and to form oxide-type ceramic bonds such as spinel bonds within the matrix by a vapor-phase reaction (Japanese Patent Application No. 60-52522).
Although the above-described techniques can increase the mechanical strength of a refractory, at the same time they increase the modulus of elasticity, so that the thermal shock resistance coefficient R ends up decreasing, and there is the problem that the thermal shock resistance is degraded.
In general, the difficulty of formation of cracks in a refractory is indicated by the thermal shock resistance coefficient R, which is given by the following formula: EQU R=S(1-.mu.)K/E.alpha.
wherein
S=rupture strength PA1 .mu.=Poisson's ratio PA1 K=coefficient of thermal conductivity PA1 E=modulus of elasticity PA1 .alpha.=coefficient of thermal expansion
Namely, the higher the rupture strength (S) is and the lower the modulus of elasticity (E) is, the better the thermal shock resistance. Therefore, it is desirable to increase the strength of a refractory without an accompanying increase in the modulus of elasticity.